Flow cytometric analysis of the total DNA/parasite of 31 clones and 2 strains of Trypanosoma cruzi demonstrates stable differences between populations about 30%. The G1, S and G2 fractions of the various clones have been correlated to their growth rate. The infection of vertebrate cells by T. cruzi and Toxoplasma gondii has been shown to be modulated by the vertebrate cell cycle implying that receptor(s) are required for this interaction and qualitative or quantitative changes in the level of these receptor(s) occur. T. cruzi clones isolated from chronic chagasic patients exhibit both inter- and intra-patient differences in their growth rates. A clone of T. cruzi has been isolated which grows only as extracellular amastigotes under routine LIl culture conditions. Both clone-specific and strain-specific T. cruzi antigens have been demonstrated to occur. Some of these antigens may correlate to disease state. The suceptibility of inbred strains of mice and the course of disease varies with the clone of the parasite. In addition, both qualitative and quantitative differences in morbidity and mortality and histopathology occur in mice infected with T. cruzi clones derived from the same source.